1. FIELD OF THE INVENTION
This invention relates to fluid loading devices and more particularly to a balancing mechanism for a loading arm used for loading crude oil or the like onto a tanker subject to rolling and pitching.
2. DESCRIPTION OF THE PRIOR ART
Crude oil, petroleum distillates or like fluid are loaded onto a tanker by means of fluid loading devices which can readily follow the movement of the ship due to wave action and take into consideration the natural rolling and pitching of the tanker as a result thereof. Such fluid loading devices permit connection and disassembly to the fluid manifold of a marine vessel such as a tanker, to be effected easily and simply, and such devices are generally simple in construction, easy to maintain and easy to inspect, etc. Two representative examples of such fluid loading devices are explained relative to the first two figures of the drawings.
Reference to FIG. 1 shows one prior art example of a liquid loading device making use of wire cable. The device comprises an inner hollow arm 1 being pivotably coupled to an outer hollow arm 2 by way of a sheave 11a which is rotatable with arm 2 and mounted for rotation on arm 1. The inner hollow arm 1 is provided with an inner balancing arm 3 which may in fact be an extension thereof, the inner hollow arm 1 being mounted for rotation about a horizontal axis at the upper end of a fixed, vertical hollow supporting member 7, which extends vertically upward and is connected to a source of liquid (not shown). The hollow supporting member 7 supports the inner hollow arm 1 for rotation about both vertical and horizontal axes by a hollow swivel joint 8. A hollow swivel joint 9 couples the outer hollow arm 2 to hollow arm 1 and permits the outer hollow arm 2 to rotate about a horizontal axis extending through the hollow swivel joint 9. A hollow swivel joint connection at 10 at the outer end of the outer hollow arm 2 permits the liquid loading device to be coupled to a tanker manifold (not shown). Balancing weights 5 and 6 are carried by the inner balancing arm 3, weight 5 being concentrically mounted to the outer end of the inner balancing arm 3 beyond sheave 12a. Sheave 12a is mounted for pivoting about a horizontal axis extending through the inner balancing arm 3 and at right angles to the longitudinal axis of that arm. A balancing weight 6 is fixed to the sheave 12a by way of an outer balancing arm 4 which acts as a radial extension of the sheave 12a. Thus, weight 6 rotates at the outer end of balancing arm 4 about the horizontal axis extending through sheave 12a. The rotation of sheave 12a is controlled by the rotation of sheave 11a which in turn is controlled by movement of the outer hollow arm 2 relative to the inner hollow arm 1. Connection between the sheaves is achieved by joining wire cables 13a and 13b by means of fixing members 13c and 13d into an endless wire cable loop trained about sheaves 11a and 12a. The fixing members 13c and 13d fix a point on the wire cables 13a and 13b on sheaves 11a and 12a to effect rotation of one by rotation of the other.
In this construction, when the hollow swivel joint 10 connected to a manifold of a tanker (not shown) is moved with respect to the hollow supporting member 7, due to rolling and pitching of the tanker, the balancing weight 6 moves together with the outer balancing arm 4 by movement of sheaves 11a, 12a and wires cables 13a, 13b, to establish a weight balancing relationship in terms of the equation: EQU Wo(Ro sin .theta..sub.1 + Li sin .theta..sub.2) + Wi(Ri sin .theta..sub.2) - Wci (Rci sin .theta..sub.2 - Wco (Lc sin .theta..sub.2 + Rco sin .theta..sub.3) = 0
where
Wo = weight of outer hollow arm 2, hollow swivel joint 10, etc., PA1 Wi = weight of inner hollow arm 1, sheaves 11a, 12a, inner balancing arm 3, etc., PA1 Wco = weight of balancing weight 6, outer balancing arm 4, etc., PA1 Wci = weight of balancing weight 5 etc., PA1 Ro = distance between the pivot axis of outer hollow arm 2 to the gravitational center of Wo, PA1 Li = length of inner hollow arm, PA1 Lc = center distance between the pivot axis for the hollow swivel joint 8 and that of sheave 12a, PA1 Ri = distance from the pivot axis of hollow swivel joint 8 to the gravitational center of Wi, PA1 Rci = distance from the swivel joint 8 to the gravitational center of Wci, PA1 Rco = distance from the pivot axis of sheave 12a and outer balancing arm 4 to the gravitational center of Wco.
The construction illustrated in FIG. 1 is such that an equilibrium is maintained by satisfying the relationship .theta..sub.1 - .theta..sub.2.
The advantage of this prior art device is that since the maximum angle included between the inner hollow arm 1 and the outer hollow arm 2 is 180.degree., a wide operating range of the loading arm is obtained. On the other hand, the disadvantage thereof is that an elongation is produced in the wire cable array when it is loaded causing difficulty in maintaining equilibrium and it requires maintenance such as periodic coating with an anticorrosive material, etc. Besides the construction of FIG. 1, such devices are constructed employing sprocket wheels instead of sheaves 11a, 12a and endless chains are used instead of wire cables such as 13a, 13b, but the elongation due to wear and the disadvantageous maintenance are not materially reduced.
FIG. 2 shows a second example of a prior art fluid loading device using instead of the cable array and sheaves, a single connection rod, and wherein parts identical to those shown in FIG. 1 are given like numerical designations. Thus, the parts designated by numerals 1-10 and the construction thereof are substantially the same as that shown in FIG. 1. In FIG. 2, an arm 11b which is rotatably mounted to follow the rotation of the outer hollow arm 2 relative to the inner hollow arm 1, has pivotably connected thereto, at one end, a single connecting rod 13e which in turn is pivoted to the outer end of a balancing arm 4 and which constitutes a parallel link to the inner hollow arm 1. It is so arranged that equilibrium is maintained as in the case of FIG. 1 by movement of the balancing weight 6 coupled also to the outer end of arm 4 by the connection through connecting rod 13e, and arm 11b follows movement of the hollow outer arm 2.
In the device of FIG. 2, the maintenance and inspection may be effected in an easier manner as compared with the case of the wire and cable arrangement of the device shown in FIG. 1. On the other hand, it is necessary to maintain the maximum included angle within a limit in order to restrict the load applied to the connection rod 13e. Furthermore, when the outer hollow arm 2 is retracted inwardly, as shown at the right in FIG. 2, a large compressive force acts on the connecting rod 13e which operates to balance the rotating force of the outer hollow arm 2 and the outer balancing arm 4 due to their own weights so that a further drawback occurs in the prior art device of FIG. 2 in that it is necessary to provide a connecting rod 13e with a relatively large cross sectional area to provide the necessary buckling strength due to the effects of the compressive force acting thereon.